There has heretofore been a known electrophotographic technique in which a photoconductive layer deposited on an electrode layer is fully charged and then subjected to image exposure, and the electric charge in the exposed regions is leaked, thereby optically forming an electrostatic latent image on the photoconductive layer, and thereafter toner having electric charge which is opposite in polarity to the residual electric charge is allowed to adhere thereto, thereby developing the image on paper or the like by electrostatic transfer. This technique is mainly employed for copying purposes. The electrostatic charge retaining period in the photoconductive layer as a recording medium is short, and toner development is carried out immediately after the formation of an electrostatic latent image. This technique cannot be used for other purposes, for example, photographing, because of low sensitivity.
In the meantime, an electrostatic information recording method by exposure carried out under voltage application has been developed in which an electrostatic information recording medium is disposed face-to-face with a photosensitive member having a photoconductive layer provided on an electrode, and image exposure is carried out with a voltage being applied between the respective electrodes of the photosensitive member and the recording medium, thereby recording an electrostatic latent image of extremely high resolution on the electrostatic information recording medium. It is extremely important that the electrostatic information recording medium used in this electrostatic information recording method should have excellent electric charge retaining characteristics.
As resins used to form an electric charge retaining layer, fluorocarbon resins are superior in electric charge retaining characteristics but unfavorable in terms of processability required therefor when layered on the electrode. As insulating materials, fluorocarbon resins exhibit high insulation properties with respect to electrons. However, in the case of tetrafluoroethylene-hexafluoropropylene copolymer (FEP), for example, the mobility of electrons is not higher than 10.sup.-17 cm.sup.2 /V.s, whereas the mobility of holes is as high as 2.times.10.sup.-9 cm.sup.2 /V.s. Thus, the insulation properties with respect to holes cannot be said to be satisfactory.
On the other hand, polystyrene resins are superior in that the glass transition temperature is high and the moisture absorption is low. However, the electric charge retaining characteristics of these resins are so low that none of them can be used as a material for an electric charge retaining layer.
It is an object of the present invention to provide an electrostatic information recording medium having an electric charge retaining layer which is excellent in electric charge retaining characteristics, particularly in positive electric charge retaining performance, and also excellent in heat resistance, moisture resistance and processability.